Cancers are the second most prevalent cause of death in the United States, causing 450,000 deaths per year. One in three Americans will develop cancer, and one in five will die of cancer. While substantial progress has been made in identifying some of the likely environmental and hereditary causes of cancer, there is a need for substantial improvement in the diagnosis and therapy for cancer and related diseases and disorders.
A number of so-called cancer genes, i.e., genes that have been implicated in the etiology of cancer, have been identified in connection with hereditary forms of cancer and in a large number of well-studied tumor cells. Cancer genes are broadly classified into “oncogenes” which, when activated, promote tumorigenesis, and “tumor suppressor genes” which, when damaged, fail to suppress tumorigenesis. While these classifications provide a useful method for conceptualizing tumorigenesis, it is also possible that a particular gene may play differing roles depending upon the particular allelic form of that gene, its regulatory elements, the genetic background and the tissue environment in which it is operating. Human epidermal growth factor receptor 3 is one of these genes implicated in the etiology of cancer (see, e.g. Munster et al., Cancer Res. Jun. 1, 2002; 62(11):3132–7; Menard et al. J Cell Physiol 2000 February; 182(2):150–62; Basso et al., Oncogene 2002; Feb. 14, 21(8):1159–66; and Yarden Oncology 2001; 61 Suppl 2:1–13).
Human epidermal growth factor receptor 3 (HER3) (see, e.g. Kraus et al., (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9193–9197) is a member of the type I receptor tyrosine kinase (RTK) family, which also includes EGFR, HER2/neu, and HER4 (see, e.g. Ullrich et al., (1984) Nature 309, 418–425; Schechter et al., (1985) Science 229, 976–978; Plowman et al., (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 1746–1750). HER3 forms heterodimers with other members of the type I RTK family, including the HER2/neu receptor (see, e.g. Sliwkowski et al., (1994) J. Biol. Chem. 269, 14661–14665; Fitzpatrick et al., (1998) FEBS Lett. 431, 102–106; Heldin, C. H. (1995) Cell 80, 213–223; Tzahar et al., (1997) EMBO J. 16, 938–4950). The HER2/neu receptor is amplified and overexpressed in 25–30% of human breast and 8–11% off human ovarian cancers. This overexpression correlates with increased morbidity and mortality, and there is evidence that the overexpressed HER2 receptor leads to aggressive malignancies (see, e.g. Slamon et al., (1987) Science 235, 177–182; Slamon et al., (1989) Science 244, 707–712; Plowman et al., (1993) Nature 366, 473–475; Dougall et al., (1996) DNA Cell Biol. 15, 31–40).
Cells expressing only HER2 receptors alone and not other members of the EGFR family fail to bind heregulin, but HER2/neu has high tyrosine kinase activity. HER3 is a kinase defective receptor (see, e.g. Guy et al. (1994) Proc Natl Acad Sci USA 91(17), 8132–6), but has binding affinity for heregulin (see, e.g. Carraway et al. (1994) J Biol Chem 269(19), 14303–6). The HER2/HER3 heterodimer forms a high affinity heregulin receptor with tyrosine kinase activity. Heregulin binding to cells that display the HER2/HER3 heterodimer causes a mitogenic response both in vitro and in vivo, so understanding this interaction is of medical importance (see, e.g. Aguilar et al., (1999) Oncogene 18, 6050–6062; Sliwkowski et al. (1994) J Biol Chem 269(20), 14661–5; Heldin, C. H. (1995) Cell 80(2), 213–23; Tzahar et al. (1997) Embo J 16(16), 4938–50). Alternate transcripts of HER3 isolated from an ovarian carcinoma-derived cell line have been identified which encode truncated forms of the extracellular domain of HER3, including three clones where the protein products were soluble secreted proteins (see, e.g. Lee, H., and Maihle, N. J. (1998) Oncogene 16(25), 3243–52). A naturally occurring secreted form of HER3 has been found to inhibit heregulin-stimulated activation of HER3 (see, e.g. Lee et al. (2001) Cancer Res 61(11), 4467–73). This provides evidence that HER3 could be an important target in breast cancer therapy.
Type I receptor tyrosine kinases typically contain four extracellular domains, a single hydrophobic transmembrane segment, and a cytoplasmic tyrosine kinase domain (see, e.g. Ullrich, A., and Schlessinger, J. (1990) Cell 61, 203–212). HER2/neu is a very active tyrosine kinase, but cells expressing HER2/neu alone, and not other members of the EGFR family, fail to bind heregulin. Conversely, the HER3 receptor binds heregulin but has low tyrosine kinase activity (see, e.g. Guy et al., (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 8132–8136; Carraway et al., (1994) J. Biol. Chem. 269, 14303–14306). As noted above, the HER2/HER3 heterodimer is a high affinity heregulin binding complex with signaling activity through the HER-2 kinase domain. To date, the domains of HER3 involved in ligand binding and heterodimerization have not been identified.
Thus far, the high carbohydrate content (see, e.g. Horan et al., (1995) J. Biol. Chem. 270, 24604–24608) and the relatively large size (˜180 kDa) of the receptors in the EGFR family have frustrated structural analysis by x-ray crystallography and NMR, so other methods have been sought to illuminate the structure and function of HER3. The extracellular domains (ECDs) of the type I RTKs have been divided into four domains: I, II, III, and IV, based on sequence analysis (see, e.g. Yarden, Y., and Ullrich, A. (1988) Annu. Rev. Biochem. 57, 443–478). Domains II and IV are cysteine-rich and are similar in sequence. Domains I and III also have sequence similarity (see, e.g. Yarden, Y., and Ullrich, A. (1988) Annu. Rev. Biochem. 57, 443–478; Lax et al., (1988) Mol. Cell. Biol. 8, 1970–1978). Little is known about the specific function of each domain except in EGFR, where several lines of evidence provide evidence that the major determinants for EGF binding lie in domain III. These lines of evidence include the following: 1) the exchange of domain III in chicken EGFR for domain III from human EGFR confers binding of human EGF (see, e.g. Lax et al., (1989) EMBO J. 8, 421–427; Lax et al., (1991) Cell Regul. 2, 337–345); 2) monoclonal antibodies that recognize residues in domain III prevent EGF binding to EGFR (see, e.g. Wu et al., (1989) J. Biol. Chem. 264, 17469–17475); 3) cross-linking of EGF to EGFR identified residues in domain III (see, e.g. Summerfield et al., (1996) J. Biol. Chem. 271, 19656–19659; Wu et al., (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3151–3155); and 4) limited proteolysis of the ECD of EGFR produced a fragment that encompassed domain III, which bound transforming growth factor, with the observation that binding could be enhanced by including portions of domain IV (see, e.g. Kohda et al., (1993) J. Biol. Chem. 268, 1976–1981). Additional studies from cross-linking experiments indicated that bound EGF is also close to tyrosine 101 in domain I of murine EGFR (see, e.g. Woltjer et al., (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7801–7805). Taken together, these experimental results provide evidence that domain I and III are close to the ligand-binding region in EGFR and that domain III contributes most of the binding. HER3 and EGFR have relatively high sequence identity (45% identity in the ECD) and belong to the same family of type 1 tyrosine kinase receptors; however, they bind a different subset of ligands and differ in preference for heterodimerization versus homodimerization (see, e.g. Tzahar et al., (1997) EMBO J. 16, 938–4950; Lemmon et al., (1997) EMBO J. 16, 281–294; Huang et al., (1998) Biochem. J. 331, 113–119; Alimandi et al., (1995) Oncogene 10, 1813–1821).
While the existing art provides a limited understanding of the structure of HER3 and the interaction between heregulin and HER3, this art does not delineate the domains in HER3 responsible for interacting with heregulin. Consequently there is a need in the art for the identification and characterization of the domains in HER3 involved in this interaction so that methods and materials for modulating this interaction can be generated. The disclosure provided herein meets this need.